Split aperture array for increased short range target coverage

ABSTRACT

A phased array radar system comprising a plurality of radiating elements configured in a common array aperture for detecting and tracking targets; and a transmit and receive arrangement responsive to a first control signal for configuring the plurality of radiating elements to define a plurality of sub-apertures from the common array aperture for detecting and tracking short range targets, wherein the plurality of sub-apertures are independently steerable array apertures.

FIELD OF INVENTION

The present invention relates generally to radar systems and morespecifically to a system and method for enabling short range arrayantenna coverage without significantly impacting long range performance.

BACKGROUND

The detection and tracking of a target object or objects is typicallyaccomplished with radio detection and ranging, commonly known as radar.Radar systems typically emit electromagnetic energy and detect thereflection of that energy scattered by a target object. By analyzing thetime difference of arrival, Doppler shift, and various other changes inthe reflected energy, the location and movement of the target object canbe calculated. Phased array antenna systems employ a plurality ofindividual antenna elements or subarrays of antenna elements that areseparately excited to cumulatively produce a transmitted electromagneticwave that is highly directional. The radiated energy from each of theindividual antenna elements or subarrays is of a different phase,respectively, so that an equiphase beam front or cumulative wave frontof electromagnetic energy radiating from all of the antenna elements inthe array travels in a selected direction. The differences in phase ortiming among the antenna activating signals determines the direction inwhich the cumulative beam from all of the individual antenna elements istransmitted. Analysis of the phases of return beams of electromagneticenergy detected by the individual antennas in the array similarly allowsdetermination of the direction from which a return beam arrives. Suchprocessing as described above is well known to those of ordinary skillin the art.

A pulse based radar system scans a field of view and emits timed pulsesof energy. Such radar systems, including, for example, CTA type radarsystems, can require both short range and long range target detectionand tracking. Long range (e.g. on the order of 60 kilometers (Km) ormore) detection performance requires relatively long pulse repetitionintervals (PRI). A narrow beam is typically required for long rangetarget detection and tracking.

A conventional radar array having a plurality of radiating elements 100configured to define an array aperture A for generating a narrow beamfor long range detection and track performance is depicted in FIG. 1.The longer PRI reduces the probability of detecting high verticalvelocity, shorter range targets (e.g. targets within about 15 Km). Inorder to alleviate this problem, systems may utilize separate shortrange (SR) and long range (LR) pulses in an attempt to cover all targetranges. However, even with SR pulses, significant limitations exist inconventional radar systems processing and implementation.

For example, short range detection and localization performance ofconventional radar systems is typically not limited by targetsignal-to-noise ratio (SNR), but rather by the number of lookopportunities of the target by the radar. This number is limited by suchfactors as high target vertical velocities, elevation beamwidth, andtarget revisit rate. More specifically, short range target detection andlocalization is usually not a function of SNR, because such short rangetargets typically have SNRs well in excess of typical thresholddetection levels. However, the problem lies with the number of lookopportunities with which to detect, track and localize a target withsufficient accuracy to evaluate a projectile launch or impact point. Aradar system utilizing a narrow beam long range pulse for detecting andtracking targets may operate quite effectively for long range objects;however, such a system may be inadequate to track short range objectshaving high target vertical velocities, which require much greaterprocessing and response time, but which does not require such narrowbeam(s). Alternative techniques for detecting and tracking both longrange and short range targets within a single radar system is desired.

SUMMARY OF THE INVENTION

The invention relies in part on recognition of the problem, and inproviding a solution for enhancing short range target coverage withoutsignificantly impacting long range performance in a radar array system.

The invention takes advantage of the signal excess characteristic ofshort range targets to effectively increase the number of lookopportunities for such short range targets.

According to an aspect of the present invention, a radar array having aplurality of configurable radiating elements, which plurality defines acommon aperture A of the array, is electronically segmented or sectoredinto two independently steerable arrays, each having an aperturesubstantially half of the common aperture A. The two independentlysteerable arrays are configured vertically one on top of the other andprocessed separately in order to effectively double the elevationbeamwidths in both the transmit and receive beams without the need tospoil the beam, while also doubling the number of beams that can beused, thereby effectively doubling the target revisit rate.

In one aspect of the invention, a phased array radar system having aplurality of radiating elements configured in a common array aperture ofm×n elements comprises a controller responsive to a control signal forconfiguring a first subset of the elements of the common array apertureinto a first sub-aperture for providing a first interrogating beam; andfor configuring a second subset of the elements of the common arrayaperture distinct from the first subset, into a second sub-aperture forproviding a second interrogating beam. Each of a pair of transmitters iscoupled to a corresponding one of the first and second sub-arrays andresponsive to the controller for generating the first and secondinterrogating beams, the beams being independently steerable. A pair ofreceiver beamformers, each coupled to a corresponding one of the firstand second sub-arrays, provides beamformed output data indicative oftarget reflections from each of the first and second interrogatingbeams. A signal processor is responsive to the beamformed output datafor determining target detection and tracking according to eachsub-array.

According to another aspect, in a phased array radar system having aplurality of radiating elements configured in a common array aperture ofm×n elements, a method for detecting and tracking targets comprises:electronically segmenting the common array aperture of m×n elements intoa first sub-aperture comprising a first subset of the m×n elementsdefining a first sub-array, and a second sub-aperture comprising asecond subset of the m×n elements distinct from the first subsetdefining a second sub-array; in a first mode, generating a firstinterrogating beam from the first sub-array, and a second interrogatingbeam from the second sub-array, the first and second interrogating beamsdiffering in at least one of frequency, phase, and beam pointingdirection; independently steering the first and second interrogatingbeams; receiving beamformed data from each of the first and secondsub-apertures corresponding to the interrogating beams, andindependently processing the beamformed data to provide targetinformation for each of the first and second sub-apertures; and in asecond mode, coherently operating the first and second sub-arrays togenerate a narrowband interrogating beam of a given frequency; receivingbeamformed data from each of the first and second sub-aperturescorresponding to the narrowband interrogating beam, and combining thebeamformed data to provide target information indicative of the fullcommon array aperture.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding of the present invention will be facilitated byconsideration of the following detailed description of the preferredembodiments of the present invention taken in conjunction with theaccompanying drawings, in which like numerals refer to like parts, andwherein:

FIG. 1 a is a front view of an aperture of a phased array antennasystem.

FIG. 2 is a simplified block diagram of a split aperture arrayconfiguration useful for short range target detection and trackingaccording to an embodiment of the present invention.

FIG. 3 is a simplified block diagram of a split aperture arrayconfiguration useful for long range target detection and trackingaccording to an embodiment of the present invention.

FIG. 4 is a schematic illustration of two electronically steerablearrays and associated beams corresponding to the embodiment of FIG. 2useful for short range detection and tracking.

FIG. 5 is a schematic illustration of a narrow beam corresponding to theembodiment of FIG. 3 useful for long range detection and tracking.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding, while eliminating, for the purpose ofclarity, many other elements found in radar systems and methods ofmaking and using the same. Those of ordinary skill in the art mayrecognize that other elements and/or steps may be desirable inimplementing the present invention. However, because such elements andsteps are well known in the art, and because they do not facilitate abetter understanding of the present invention, a discussion of suchelements and steps is not provided herein.

Referring now to FIG. 1 a, there is shown a front view of a phase arrayantenna system 100 having an aperture A shown by way of example and notlimitation, and including a rectangular array of m×n antenna elements101 arranged in rows and columns. The antenna elements are eachassociated with respective transmit/receive (T/R) modules (not shown).Such radiating elements may be dipoles, monopoles, and/or other suchradiators as is understood in the art. Each T/R module or elementprovides the active transmit/receive electronics required to operate theantenna element in transmit and receive mode. In an exemplaryembodiment, each T/R module comprises a circulator coupled to a variableattenuator or amplitude shifter via low noise receive amplifier. A phaseshifter may be switchably coupled via a T/R switch to transmit to a highpower amplifier or to a variable attenuator for operation in either atransmit or receive mode of operation. The phased array of antennaelements are configured in a regular, periodically spaced grid asillustrated in FIG. 1 a.

It is understood that such a radar system is known in the art. Many suchradar systems are known, including for example, the system as depictedin U.S. Pat. No. 6,084,540, entitled “Determination of Jammer DirectionsUsing Multiple Antenna Beam Patterns” assigned to Lockheed MartinCorporation, the assignee herein, the subject matter thereofincorporated herein by reference. Therein, an array antenna receivesantenna beam pattern control signals over a signal path, for controllingthe beam pattern. Those skilled in the art know that the beam pattern ofan array antenna can be controlled to produce one or more directedbeams, which may be broad, or of the narrow band or “pencil” type. Sincecontrol of an array antenna does not involve moving any physical object,control of the beam direction can take place almost instantaneously.Consequently, multitudes of directional beams can be generated insequence in a very short period of time. As an alternative, the antennabeam controls can be adjusted to simultaneously produce multipledirectional beams. In FIG. 1 of U.S. Pat. No. 6,084,540, threerepresentative beam patterns are illustrated, which point in differentdirections from the antenna, as might be desirable when trackingmultiple radar targets. The directional patterns each include aplurality of sidelobes. Each sidelobe is separated from the adjacentsidelobe, and from any adjacent main lobe, by a null in the antenna orbeam pattern.

Referring now to FIG. 2, there is shown a block diagram of an exemplarysplit aperture array configuration 200 for short range target detectionand tracking according to an embodiment of the present invention. Asillustrated therein system control function module 210 includesprocessor control logic for generating array control commands forcontrolling transmit and receive functions of T/R modules or elements101 in the phased array antenna assembly 100 on a per-element basis. Inone embodiment, radar array 100 has a plurality of radiating elements101 configured in a common array aperture A of m×n elements. When thesystem is to be operated in a short range detection/tracking mode,transmit control commands are generated from control processor 210 andare provided to each of a pair of transmit modules 202, 212 coupled tothe array. Each transmit module (202, 212) includes waveform generatorand exciter circuitry for electronically separating the common arrayaperture A of m×n elements into a first sub-aperture A1 comprising afirst subset of the m×n elements, and a second sub-aperture A2comprising a second subset of the m×n elements. In a preferredembodiment, waveform generator and exciter modules operate to split thearray aperture A electronically into two sub arrays of aperture A1 andA2, one on top of the other, with A1 and A2 each equal to A/2. That is,sub aperture A1 defines a first subarray 201 of size m/2×n elements, andsub aperture A2 defines a second subarray 203 of size m/2×n elements. Ofcourse, it is understood that each of the subarrays may be segmentedinto less than one half of the full aperture common array, according tothe particular application, mode, and system requirements. Thetransmitter/exciter circuitry transmits signals to the phased arrayantenna assembly and hence to each of the subarrays for providing twoindependently steerable arrays. In a preferred embodiment, the splitaperture short range mode provides for two simultaneous beams havingtwice the beamwidth as that of a single beam formed via the full arrayaperture A enabling an increase in short range coverage of about 4:1.

Referring now to FIG. 4 in conjunction with FIG. 2, in short range modethe system operates to provide two transmit beams F1, F2, respectively,from sub-arrays 201, 203 simultaneously for short range target detectionand tracking. The transmit beams may be broader beams for increasedelevation coverage for short range targets such as missiles or otherprojectiles. The widened beams in elevation are enabled by the high SNRmargin associated with short range targets and may effectively increasecoverage by a 2 to 1 ratio. In general, the transmit or interrogatingbeams differ in at least one of frequency, phase, and beam pointingdirection, as controlled by the processor control logic 210. For shortrange (SR) pulse waveforms the number of search beams is effectivelydoubled, as twice as many beams effectively double the target revisitrate.

In an exemplary configuration, short range half aperture processing isaccomplished using an SR pulse width of about 1 to 10 microseconds (us)with a PRI of about 40 to 100 us. The pulse widths and PRI for each ofthe beams of the dual apertures A1, A2, would each be of the sameduration, but of different frequency and pointing direction, withtransmission (and subsequent reception) occurring at the same times foreach sub array. In other words, both transmit beams out from aperturesA1, A2 would be output at the same time, and both receive beams would bereceived by the separate beamforming circuits at the same time.

Still referring to FIG. 2, for receive beam processing, reflected signaldata is received via each of apertures A1 and A2 and separatelyprocessed by receiver circuitry modules 204, 214, respectively.Beamformer signal outputs from each sub-array are down converted via anRF downconverter arrangement, A/D converted into digital form, andapplied separately to produce desired beams. The signals representingthe various beams are applied to signal processor logic 206, 216 whichperforms target signal detection and location processing, weightcalculations (including, e.g. adaptive weight calculations), antennanulling, and other signal processing of the received waveforms as isunderstood by those of ordinary skill in the art. Signal processor logicmay be operatively coupled to one or more memory units (not shown) forstoring, retrieving and processing array information includingcalibration data in the form of mutual coupling coefficients, dynamicrange and SNR data, transmit power and received signal strength, forexample. The beamformer and signal processor modules may also include orbe operatively coupled to signal detection circuitry and functionalityfor detecting and processing the transmitted/received signals, includingdetection of null conditions and threshold comparisons.

Control Processor 210 may also include or be operatively coupled toperformance monitoring and fault detection circuitry for processing andidentifying failed or degraded elements for later maintenance orreplacement.

The output of signal processor modules 206, 216 are fed into dataprocessor logic 208, 218, which operate to perform target detection andlocation processing of the target data associated with each of the subapertures A1, A2, and fed to a display unit 220 for displaying theinformation to a user.

The beamformer receiver in general provides for the application of phaseshifts to each element (via phase shifters), and then sums the result.Further filtering and analog to digital (A/D) conversion may also beincluded. The signal processor will operate on this digital data tofurther filter the signal as needed, perform pulse compression, Dopplerfiltering, magnitude detection, and thresholding for target detection asis well known to those skilled in the art. The data processor coupled tothe signal processor will use this target detection data to formtrackers which track the targets and determine target characteristics,such as trajectory, and launch and/or impact points as is well known tothose skilled in the art. The control processor serves to coordinate thefull and half aperture modes by providing the appropriate controlfunctions to the array elements and the transmit/receive processing.This will include the proper phase shifts to each element duringtransmit and receive when transmitting and receiving the full aperture(long range) pulse or sub-aperture (short range) pulse as is understoodby those skilled in the art.

The separately controlled arrays and separate receiver processing enablepartial aperture (i.e. A1, A2) performance to be obtained. In apreferred embodiment, different transmit beam frequencies are utilizedfor each sub-aperture.

By splitting the array into two separate apertures, the systemexperiences a performance loss of about 9 dB. This results from about a3 db loss from each of the widened transmit and receive beams, and a 3dB loss in power. However, for short range targets, SNR is typicallyhigh due to the relatively short distances involved, thereby mitigatingproblems associated with power loss.

In accordance with another aspect of the present invention, for longrange target detection and tracking, the two split apertureconfiguration can be used coherently to form a narrower beam for longrange application without suffering long range performance degradation.FIG. 3 illustrates the configuration for operating the radar system ofthe present invention in a long range target detection and track mode.Like reference numerals depicted in FIG. 2 are used to indicate likeparts. In long range mode, control module 210 operates to control eachof the transmitters 202, 212 coherently to enable effectively fullaperture (i.e. aperture A) performance.

In an exemplary embodiment, and referring now to FIG. 3 in conjunctionwith FIG. 5 in long range mode, signal data corresponding to the samefrequency, phase and beam pointing direction is applied to bothtransmitter modules 202, 212 via controller 210 so that theinterrogating beams output from each of apertures A1, A2 coherently forma narrow beam F3 (FIG. 5) for long range detection and trackperformance.

In an exemplary configuration, long range full aperture processingoccurs using an LR pulse width of about 20 microseconds (us) or greater,with a PRI greater than about 200 us. The transmit beams may begenerated using phase locked loops, or may be generated via directdigital conversion, for example. The transmitters are coherent in timeand properly phased. A common clock or clock signal may operate to driveeach of the separate transmitters such that the exciters are in lockstep with one another. The exciter modules associated with each of thetransmitters blocks 202, 212 are then activated simultaneously. The twoexciter modules produce essentially the same waveforms which arecontrolled to the same phase, and the antenna elements are appropriatelyphased so that the apertures A1, A2 operate as a single full aperture A.It is understood that phasing of each of the antenna elements is withinthe skill of one of ordinary skill in the art, and further descriptionis omitted for brevity. Similar operation occurs for the receive beamprocessing wherein the elements are phased and the output of eachsubarray is combined (e.g. added) to produce an output associated with afull array aperture A. Because the operation is a linear operation, thesignal processing adds the output from each beamformer together toprovide a received signal associated with the full array.

The long range (LR) pulse waveforms enable long range detection andtracking using the full aperture A of the phased array antenna. As inthe short range mode, the reflected signal data is received via each ofapertures A1 and A2 and separately processed by receiver circuitrymodules 204, 214. Beamformer signal outputs from each sub-array are downconverted via an RF downconverter arrangement, A/D converted intodigital form, and applied separately to produce desired beams. Thesignals representing the various beams are then coherently combined fromsignal processor logic 206, 216 to form full aperture array beam signaland applied to data processor detector and tracking module 208. Theoutput of data processor 208 is fed to a display unit 220 for displayingthe information to a user.

As identified in FIGS. 2 and 3, processor 210 operates in conjunctionwith memory 148 which comprises an operating system that contains thevarious execution commands necessary to control the arrayhardware/software and its operation. In addition, the processor andmemory include functionality selection adapted to automatically selector transition to a given mode of operation in response to a controlsignal or user input, and perform the processing steps associated withthe long range and short range techniques described herein.

Furthermore, it is understood that the control processor implementingone of long range or short range waveform and target detection/trackingmay switchably couple to the signal processor arrangement as depicted inFIG. 3 for coherently combining the beamformer output data to providefull aperture array data according to a first switch position (S1), ordecouple the signal processor arrangement as depicted in FIG. 2 forseparate parallel processing of the beamformer output data to providepartial aperture data via a second switch position (S2). Such switchingor reconfiguration may be embodied as one or more mechanical,electrical, software, or logically related switching functionsassociated with configuring the antenna system according to the requiredoperational mode of the array.

The processor, memory and operating system with functionality selectioncapabilities can be implemented in software, hardware, firmware, or acombination thereof. In a preferred embodiment, the processorfunctionality selection is implemented in software stored in the memory.It is to be appreciated that, where the functionality selection isimplemented in either software, firmware, or both, the processinginstructions can be stored and transported on any computer-readablemedium for use by or in connection with an instruction execution system,apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions.

Further, it is understood that the subject invention may reside in theprogram storage medium that constrains operation of the associatedprocessors(s), and in the method steps that are undertaken bycooperative operation of the processor(s) on the messages within thecommunications network. These processes may exist in a variety of formshaving elements that are more or less active or passive. For example,they exist as software program(s) comprised of program instructions insource code or object code, executable code or other formats. Any of theabove may be embodied on a computer readable medium, which includestorage devices and signals, in compressed or uncompressed form.Exemplary computer readable storage devices include conventionalcomputer system RAM (random access memory), ROM (read only memory),EPROM (erasable, programmable ROM), EEPROM (electrically erasable,programmable ROM), flash memory, and magnetic or optical disks or tapes.Exemplary computer readable signals, whether modulated using a carrieror not, are signals that a computer system hosting or running thecomputer program may be configured to access, including signalsdownloaded through the Internet or other networks. Examples of theforegoing include distribution of the program(s) on a CD ROM or viaInternet download.

The same is true of computer networks in general. In the form ofprocesses and apparatus implemented by digital processors, theassociated programming medium and computer program code is loaded intoand executed by a processor, or may be referenced by a processor that isotherwise programmed, so as to constrain operations of the processorand/or other peripheral elements that cooperate with the processor. Dueto such programming, the processor or computer becomes an apparatus thatpractices the method of the invention as well as an embodiment thereof.When implemented on a general-purpose processor, the computer programcode segments configure the processor to create specific logic circuits.Such variations in the nature of the program carrying medium, and in thedifferent configurations by which computational and control andswitching elements can be coupled operationally, are all within thescope of the present invention.

As shown and described herein, the present invention provides for longrange detection and localization performance of a full aperture array Awhile providing a 4:1 increase in target coverage for short rangetargets or projectiles. The present invention takes advantage of the SNRmargin for short range targets and widens transmit and receive beams inelevation to increase coverage by 2:1 in short range mode, whiledoubling the number of search beams for short range waveforms, therebyquadrupling short range target coverage. By implementing the splitaperture parallel processing configuration and SR waveform pulses forshort range detection/track, and long range coherent narrow band singlebeam processing configuration for LR waveform pulses and long rangedetection/track, baseline templates are not impacted, while providingtwice the number of short range beams in the same amount of time. Theincreased coverage for SR targets will also allow more track-while-scanprocessing to avoid impact to the timeline by reducing the number ofdedicated track beams necessary to verify/track targets.

By way of example only, a radar system embodying the principles of thepresent invention may be operated as follows. A series of: (a) eight SRbeam transmission bursts are sequenced at given time intervals at givenazimuths, followed by: (b) a single LR beam transmission at a givenazimuth. The sector to be scanned is then swept by repetitive processingof the above steps (a) and (b) until the entire sector (e.g. +/−45degrees) is scanned. Each of the SR beam transmission sequences consistsof simultaneous transmissions of an SR pulse of a given PRI from eachexciter in transmitter 202 and transmitter 212, respectively, followedby subsequent simultaneous receiver beamforming reception viabeamformers 204, 214, respectively (and subsequent signal and dataprocessing) for partial array aperture processing associated with eachSR beam. Each LR beam transmission consists of an LR pulse of a givenPRI simultaneously and coherently transmitted by each of transmitters202, 212, to effectively form a single narrow beam, followed bysubsequent simultaneous receiver beamforming reception via beamformers204, 214, respectively (and subsequent signal and data processing) forfull aperture array processing. This 8 SR/1 LR sequencing, systemconfiguration and processing, continues until the entire sector isscanned. In this manner, both LR and SR targets are detected, trackedand localized within a single radar having a common array aperture. Ofcourse, other sequencing and repetition rates are contemplated.

The present invention finds application in various radar array systemsand subsystems, including, for example, CTA-type radar systems andcounter-fire radars that provide or require simultaneous long and shortrange capabilities.

While the present invention has been described with reference to theillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments, as well as other embodiments of the invention, will beapparent to those skilled in the art on reference to this description.It is therefore contemplated that the appended claims will cover anysuch modifications or embodiments as fall within the true scope of theinvention.

1. A plurality of radiating elements configured in a common arrayaperture for detecting and tracking targets in a long range mode or ashort range mode of operation; a control processor operatively coupledto a pair of exciter and receiver arrangements coupled in parallel tosaid array, said control processor responsive to a control signal forconfiguring said plurality of radiating elements to define a pair ofsub-apertures from said common array aperture for detecting and trackingtargets, wherein said pair of sub-apertures are each independentlysteerable array apertures, each subaperture communicating with arespective one of said pair of exciter and receiver arrangements;wherein said short range mode, each of said exciter pairs is responsiveto a corresponding pulse waveform to form separate beams from saidsubapertures differing in at least one of frequency, phase, and beampointing direction; wherein in said long range mode, each of saidexciter pairs is responsive to a corresponding pulse waveform of thesame frequency, phase, and beam pointing direction to coherently combinesignal data to form a single narrowband beam from said subapertures. 2.The system of claim 1, wherein in said short range mode said pair ofsub-apertures comprises a first aperture for providing a beam at a firstfrequency, and a second aperture for providing a beam at a secondfrequency.
 3. The system of claim 1, further comprising a display fordisplaying output data associated with said detected and tracked shortrange targets.
 4. The system of claim 1, wherein said short range targetdistance is less than about 15 kilometers.
 5. A phased array radarsystem having a plurality of radiating elements configured in a commonarray aperture of m×n elements, said system comprising: a controller forconfiguring a first subset of said elements of said common arrayaperture into a first sub-aperture for providing a first interrogatingbeam; and a second subset of said elements of said common array aperturedistinct from said first subset, into a second sub-aperture forproviding a second interrogating beam; a pair of transmitters, each onecoupled to a corresponding one of said first and second sub-arrays andresponsive to said controller in a first mode for simultaneouslygenerating said first and second interrogating beams, said beams beingindependently steerable; a pair of receiver beamformers, each onecoupled to a corresponding one of said first and second sub-arrays forproviding beamformed output data indicative of target reflections fromeach of said first and second interrogating beams; a signal processorarrangement responsive to said beamformed output data for determiningtarget detection and tracking according to each said sub-array; whereinsaid controller is operable for providing in said first mode a signalwaveform to said pair of transmitters differing in at least one offrequency, phase, and pointing direction to generate said first andsecond interrogating beams; and wherein said controller is operable forproviding in a second mode a signal waveform to said pair oftransmitters of the same frequency, phase, and beam pointing directionsuch that the interrogating beams are coherently combined into a singlenarrowband beam output from said subarrays and corresponding to the fullarray aperture.
 6. The system of claim 5, wherein the signal processorarrangement comprises a pair of digital signal processors, each coupledto a corresponding one of said pair of beamformers.
 7. The system ofclaim 6, wherein said first sub-aperture is about one half of saidcommon array aperture, and wherein beamforming data received by saidpair of digital signal processors is coherently combined for determiningtarget detection and tracking in said second mode.
 8. The system ofclaim 6, wherein in said first mode, said signal waveform comprises apulse width of about 1 to 10 microseconds and a pulse repetitioninterval of about 40 to 100 microseconds.
 9. In a phased array radarsystem having a plurality of radiating elements configured in a commonarray aperture of m×n elements, a method for detecting and trackingtargets comprising; electronically separating said common array apertureof m×n elements into a first sub-aperture comprising a first subset ofsaid m×n elements defining a first sub-array, and a second sub-aperturecomprising a second subset of said m×n elements distinct from said firstsubset defining a second sub-array; in a first mode, generating a firstinterrogating beam of a first frequency from said first sub-array and asecond interrogating beam of a second frequency from said secondsub-array, said first and second interrogating beams differing in atleast one of frequency, phase, and beam pointing direction;independently steering said first and second interrogating beams;receiving beamformed data from each of said first and secondsub-apertures corresponding to said interrogating beams, andindependently processing said beamformed data to provide targetinformation for each of said first and second sub-apertures; in a secondmode, coherently operating said first and second sub-arrays to generatea narrowband interrogating beam of a given frequency corresponding tothe full common array aperture; receiving beamformed data from each ofsaid first and second sub-apertures corresponding to said narrowbandinterrogating beam, and combining said beamformed data to provide targetinformation indicative of the full common array aperture.
 10. The methodof claim 9, wherein said first sub-aperture is about one half of saidcommon aperture.
 11. The method of claim 9, wherein said generating saidfirst and second interrogating beams comprises providing a pulsewaveform differing in at least one of frequency, phase, and pointingdirection, to corresponding first and second transmitters incommunication with said respective subapertures, and wherein saidgenerating said narrowband interrogating beam comprises providing apulse waveform of the same frequency, phase, and pointing direction, tocorresponding first and second transmitters in communication with saidrespective subapertures.